Conventional burners, in widespread commercial use today, whether of the residential or commercial type, continuously combust air (oxygen) and fuel. Such burners will be referred to in this specification throughout as "continuous burners". In all such burners, combustion air (or oxygen) and fuel are metered at precise rates into a burner body where the fuel and combustion air is mixed into a combustible mixture and ignited. The combustion is stabilized and a continuous flame is propagated from the stabilization point, the air and fuel being combusted in the flame front. Such conventional burners are consistent and reliable and they are generally quiet. Further, their design, even for highly fuel efficient designs, has developed into widely accepted design principles which are universally followed to yield commercially dependable burners.
Developments in continuous burners have also led to improvements in their turndown ratio. Because turndown ratio can be expressed in different ways, as used herein, "turndown ratio" means the ability of the burner to vary its total heat output over a fixed period of time. In this area development work continues since it is desirable to produce a burner which can maintain stoichiometric to "lean" combustion over a wide turndown ratio. In conventional continuous burner design, turndown is accomplished by varying the rate at which combustion air and fuel are fed into the burner, but not the ratio therebetween which is fixed. Depending on the burner design there is an upper and lower mass flow rate at which combustion can no longer be regularly sustained and this determines the turndown ratio for any particular burner. Another turndown approach which has gained commercial acceptance is referred to as pulsed combustion which is not to be confused with pulse combustion which will be described below. In pulsed combustion, the fuel and air to the burner are periodically regulated to be on-off in variable cycles (usually controlled by microprocessors) and in this manner the total heat output over a given period of time can be regulated. Continuous burners have typical turndown ratios of 3:1 to 6:1 and in some instances have gone as high as 10:1.
In spite of their widespread use, continuous burners have limitations. The turndown ratio, even in pulsed combustion, is limited. Complete combustion is always a problem and even with so-called stoichiometric continuous burners, certain pollutants such as nitrous oxide emissions exist at a level higher than that which would theoretically exist if the combustion were instantaneous for a fixed volume of fuel and air. Inherently, both the gas and air supplied to the burner must be pressurized. Also, a conventional, continuous burner is capable of only heating the work or the environment, although in some heat treat applications the combustion air in the burner may be used to cool the work if the fuel is turned off.
An alternative to continuous combustion is a process known as pulse combustion. Pulse combustion is an old technology. One of the best known examples of a pulse combustor is the German V-1 "Buzz Bomb" used in World War II. A more recent example of a pulse combustor is the recently developed Lennox space heater which is operated as an acoustic Helmholtz resonator. The pulse combustion principle is illustrated in FIG. 1a-1d.
In FIG. 1a, the start-up of the cycle is illustrated. Combustion air 1 and fuel 2 are introduced simultaneously through a pair of flapper valves 3 which function as one-way pressure sensitive check valves. These reactants are mixed in the combustion chamber 4 and initially ignited by a spark plug 5. A rapid combustion (FIG. 1b) results which produces a pressure surge that advances upstream to slam shut the inlet valves and block off the entrance preventing further fuel and combustion air from entering the combustion chamber. At the same time, a pressure pulse travels downstream to produce a surge of the products of combustion 6 out of the exhaust duct as shown in FIG. 1b. When the products of combustion are discharged from the combustion chamber the pressure in the chamber tends to drop. Inertia causes the products of combustion in the exhaust duct to continue to flow through the discharge duct even after the explosion pressure in the combustion chamber has been dissipated. Conventional, accepted thinking is that the wave motion or pulse of the products of combustion drops the pressure in the combustion chamber below atmosphere with the result that the inlet flapper valves open causing a further mixture of air and fuel to enter the chamber as shown in FIG. 1c. The cycle is then repeated. It is also known that the mixture in FIG. 1c can be ignited from the hot gas residue of the previous cycle causing the process to be self-sustaining. The process is usually driven acoustically typically at the resonance frequency.
There are several different pulse combustor designs which all operate on the same underlying principle, i.e. the periodic addition of fuel and air must be in phase with the periodic pressure oscillations. In the literature, the pulse combustors are generally identified as the quarter wave or Schmidt tube, the Rijke tube and the Helmholtz resonator. Referring to FIG. 1a, the Lennox space heater operates as an acoustic Helmholtz resonator with its small neck replaced by a tailpipe 7. The German V-1 "Buzz Bomb" operated as a quarter wave tube in that the tailpipe as shown in FIG. 1a was shaped as an exhaust duct with combustion occurring at a distance x=length/4 which generated a thrust harnessed for propulsion. The Rijke tube is similar to the quarter wave or Schmidt tube and comprises a vertical tube open at both ends which contains a heat source in the center of its lower half, that is at x=length/4. The Rijke combustor is generally used with liquid fuel because the upward flow of heat from the heat source can be utilized to volatilize the fuel to produce the combustion at the desired location. There have been countless design variations. Generally, combustion air may be premixed with the fuel and/or fuel premixed with the air and/or a premixing chamber utilized in conjunction with the combustion chamber. Principally, gaseous fuel can be (1) premixed with entering air; (2) fed continuously to the combustion chamber; (3) supplied from a plenum through a separate aerodynamic valve; or (4) supplied from a tuned chamber. In all pulse combustors, the fuel and air quantities are mixed and then brought, more or less as a total mixture, into an explosive ignition which produces the noise associated with the devices, and generates the pulsed pressure waves which control the fuel and air combustion. Typically, flapper valves as shown in FIGS. 1a-1c simultaneously admit and mix the fuel and air as they are drawn into the combustion chamber. In the tube arrangements discussed, the air may be drawn into the tube vis-a-vis a flapper valve while the fuel is emitted downstream in the tube. The fuel and air mix as they travel further downstream to the point where the total mixture is explosively ignited and this ignition/combustion produces the noise and shock typically associated with pulse combustion.
As thus defined, pulse combustors are generally recognized to have certain advantages over the steady state combustion employed in continuous burners used in most boilers and furnaces. The advantages include:
(a) Because of the sudden combustion, pulse combustors are believed to have combustion intensities that are up to an order of a magnitude higher than conventional burners.
(b) Pulse combustors are generally believed to have heat transfer rates that are a factor of two to three times higher than continuous burners. This results because in most pulse combustors, the combustion occurs near the closed end of a tube where inlet valves operate in phase with pressure amplitude variations to produce localized temperature and pressure oscillations around a mean value. More specifically, it is known that flow oscillations can significantly increase beat transfer over steady turbulent flow and the oscillations, if large enough, can in themselves create additional turbulence increasing heat transfer. This means that more heat can be removed with a smaller more compact heat exchanger thus decreasing the overall cost of a furnace or heater.
(c) Because of the suddenness of the combustion, it is generally believed that nitrous oxide emissions are reduced or lowered by as high a factor as three.
(d) Finally, pulse combustors are inherently self-aspirating since the combustor generates a pressure boost. This obviates the need for a blower and also permits the use of a compact heat exchanger that may include a condensing section which obviates the need for a chimney or a draft, an important consideration in many applications.
While the advantages of pulse combustion when compared to conventional steady state combustion devices are significant, there are serious disadvantages associated with pulse combustion which has heretofore prevented their wide scale commercial acceptance. The disadvantages include:
(i) All pulse combustion systems produce objectionable noise whether the systems are acoustically driven or otherwise. This is inherent because the combustible mixture is formed from the complete charge which produces an explosive ignition. A typical approach which is followed to mute the noise is a system using pairs of pulsed combustors which must be operated in phase at or near resonance so that the pressure or noise from one unit cancels the noise or pressure pulse of the other. The pressure or noise is not eliminated and along with the noise is shock resulting from the explosion. The chamber and tail pipe have to be designed to withstand the shock.
(ii) The second principal defect present in current pulse combustors is the fact that they posses little if any turndown ratios. For example, acoustically driven pulse combustors operate at one combustion speed, the resonance frequency. As noted above, all pulse combustors are operated in self-sustaining phase such that the fuel and air is admitted in periodic phase relationship with the pressure oscillations resulting from the explosion of the air and fuel. This means that the entire arrangement has to simply be operated on/off to achieve turndown. Any attempt to achieve turndown by varying the charge of fuel plays havoc with the interaction of combustion chamber geometry and combustion oscillations which are precisely configured to insure sudden combustion at a fixed volume of fuel and combustion air.
(iii) Finally, and notwithstanding the commercial success of certain prior art pulse combustion systems such as the Lennox system, there is in general a reliability or consistency problem affecting prior art pulse combustion systems. As noted, the success of any pulse combustion system is critically dependent on the geometry of the combustion cavity and this geometry is presently determined by trial and error to produce a specific combustor geometry for a specific application which is characterized by a narrow turndown ratio and some form of attachment to mute the noise resulting from ignition explosion. The design parameters which permit consistently reliable pulse combustion burners to be built have not been developed.